The present invention relates to x-ray tubes and in particular to high performance targets for use in x-ray generating equipment. More particularly, the invention is directed to high performance rotating x-ray tube anode structures which reduces the loss of vacuum caused by gas formation during operation.
The use of metal targets to receive a stream of high energy electrons and to emit x-rays as a result of electron impact on the metal target has been known for many years. As this art has evolved, the targets subjected to electron bombardment to generate x-rays have had to operate at higher and higher temperatures. The development of CAT scan equipment has accelerated this demand for higher and higher operating temperatures for such targets. This is mainly because of the higher current of electrons striking the metal surface and also the frequency of electron impact on the target.
In general, the modern high performance metal x-ray tube target is a rotating anode which has three main functional parts. These are the focal track on which the electrons impact; the substrate on which the focal track is carried; and an emissive coating on the substrate.
The focal track is the area on the front face of the target towards which the electron beam is directed and which absorbs the electrons and generates the x-rays. Heat is also generated as the electrons are absorbed and the x-rays are generated. This region is commonly made up of a tungsten alloy. A powder metallurgy process is employed in forming the focal track region and this process consists of sintering, forging, and heat treating. The density of the metal of the focal track is commonly about 94% of theoretical density.
The alloy substrate beneath the focal track constitutes the bulk of the target mass. The substrate supports the focal track on its front face. The substrate takes up the heat generated in the focal track and dissipates this heat. The alloy substrate is commonly made up of molybdenum alloyed with titanium, zirconium, and carbon. It is coprocessed with the focal track alloy through the sintering, forging, and heat treating steps. An alloy which is currently preferred for this purpose has the nominal composition after processing comprising about 0.5 atom percent titanium, about 0.08 atom percent zirconium, about 250 ppm carbon and the balance molybdenum.
A thermal emittance enhancing coating is applied to the back and sides of the rotating anode to increase heat dissipation by radiation. A variety of such coatings are known. For example, U.S. Pat. No. 4,132,916 describes coatings composed of zirconium dioxide, hafnium oxide, magnesium oxide, strontium oxide, cerium dioxide and lanthanum oxide or mixtures thereof. U.S. Pat. Nos. 4,953,190; 4,029,828; and 4,090,103 also disclose emissive coatings on x-ray targets. Such coatings are generally applied by known methods such as plasma spraying followed by a vacuum or low pressure heat treatment to remove gas trapped or generated during the processing.
One of the problems which has developed in connection with the operation of high performance x-ray tubes is that the increase in the energy input demands on the target has resulted in alloy substrate bulk temperatures which have increased dramatically. At present, peak substrate temperatures reach 1300.degree. C. to 1500.degree. C. At these temperatures, a new problem has developed, namely the generation of gaseous reaction products within the vacuum tube. It is, of course, well known that x-ray equipment operates in a vacuum and that sustaining the vacuum is necessary in order to have a desirable and optimum operation of the x-ray tube itself.